Wireless power transfer system for deep-implanted biomedical devices

In this paper, a dual-band implantable rectenna is proposed for recharging and operating biomedical implantable devices at 0.915 and 2.45 GHz. The rectenna system consists of a compact dual-band antenna based on a meandered-resonator as well as efficient dual-band rectifier circuit. Both components (antenna and rectifier) are integrated inside a capsule device to simulate and experimentally validate the rectenna. The antenna occupies lower volume (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$5 \times 5.25 \times 0.25$$\end{document}5×5.25×0.25 \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\hbox {mm}^{3}$$\end{document}mm3), where compactness is achieved using meandered geometry and a slotted ground plane. It maintains quasi-omnidirectional radiation patterns and peak realized gains of −22.1 dBi (915 MHz) and −19.6 dBi (2.45 GHz); thus, its capability is enhanced to harvest the ambient energy from multiple directions. Moreover, a dual-band rectifier is designed using a dual-branch matching network (an L-matching network and open-circuited stub in each branch) with a radio frequency (RF) to direct current (DC) conversion efficiency of 79.9% for the input power of 1 dBm (lower band: 0.915 GHz) and 72.8% for the input power of 3 dBm (upper band: 2.45 GHz). To validate the concept of the rectenna, the implantable antenna and rectifier are fabricated and attached together inside a capsule device, with the measured results verifying the simulated responses. The proposed rectenna efficiently rectifies two RF signals and effectively superimposes on a single load, thus, providing a distinct advantage compared to single-band rectennas. To the best of the authors’ knowledge, this is the first-ever implantable rectenna to perform dual-band RF signal rectification.

www.nature.com/scientificreports/ In this paper, a dual-band WPT system is designed, where a wideband log periodic antenna acts as a source for the implantable rectenna. Initially, each part of the WPT system is individually designed and experimentally tested. Then, the PTE is analyzed, and a matching layer is used to enhance it. Finally, the rectifier is connected to the implantable antenna. As a proof, the log periodic antenna (the WPT transmitter) is sourced with an RF signal generator, the integrated-implantable antenna is placed inside minced pork meat and the received voltage is observed on digital multimeter (DMM). The proposed dual-band rectenna is an excellent choice for future implants due to its compact dimensions, human safety analysis, dual-band RF signals rectification, and high RFto-DC conversion efficiency. Moreover, to the best of the authors' knowledge, this is the first-ever implantable rectenna to perform dual-band RF signal rectification.

Dual-band rectifier design
Matching network and series-diode design. A rectifier circuit has the ability to obtain useful DC power from the RF sources. Figure 2 shows general topology of the proposed dual-band (915 MHz and 2.45 GHz) rectifier. An optimal frequency is needed to realize an efficient WPT system [29][30][31] . These frequency bands (ISM bands of 915 MHz and 2.45 GHz) are selected because they do not require license. Initially, the optimal frequency is set to be 915 MHz. Then, 2.45 GHz is also chosen because it is license free band. The optimal frequency is derived using the Equation (1) 32 .
where, f opt is the optimal frequency, ǫ r0 is the static permittivity, c is the speed of light, τ is the relaxation time constant and ǫ ∞ is the permittivity at the frequency where ωτ ≥1 33 . Using Equation (1), the optimum frequency is set to be 915 MHz. Later, the 2.45 GHz frequency band is chosen because it is a license free band.
This design comprises a complex matching network, a rectifying topology, an output filter, and a power management unit or load. Unlike a traditional single-band rectifier, the proposed topology uses two branches of matching networks. The matching circuit between the antenna and rectifier is important, as it determines the amount of power delivered from the antenna to the rectifier. The upper branch matches the lower-frequency ISM band (915 MHz) signal of the antenna with the rectifier, and the lower branch matches the higher-frequency ISM band (2.45 GHz) signal of the antenna with the rectifier. Considering that the complex and non-linear load of the rectifier changes with the frequency, a complex matching network is required. Therefore, the matching network consists of an L-section matching network and an open-circuited stub, as shown in Fig. 2b. The initial values of the two inductors of the L-section matching network are calculated using the following relations 34 : where L 1(l,u) and L 2(l,u) are the inductances of the matching network, ω is the angular frequency of the incoming signal, R in is the input resistance of the rectifier, R ant is the resistance of the implantable antenna, Q is the quality factor that depends on the bandwidth, and X in is the input reactance of the rectifier. It is worth mentioning that L 2(l,u) as a part of the L-section matching network is also used for DC return path. The values of input resistance ( R in ) and input reactance ( X in ) of the rectifier can be found by simulating the circuit excluding the L-section matching network.
The final values of the inductances, along with the open-circuited stub, are implemented in Keysight Advanced Design System (ADS). The simulations are performed based on two objectives: (1) optimum matching between the source and rectifier and (2) maximum RF-to-DC conversion efficiency at a wide range of input power. The lumped components are connected with one another using microstrip traces. RF Schottky diodes model HSMS-2850 are selected because of their fast switching and lower operating voltage. Initially, the ideal circuit elements of the ADS are used for optimization, but, they are ultimately replaced with muRata components in the ADS library. The optimized schematic of the proposed rectifier is designed on a Rogers RO3010 material ( ǫ r = 10.2) with a (1)   RF-to-DC efficiency of the rectifier. Matching networks are designed so as to maximize the available power for the rectifying section. It is worth mentioning that the matching network minimize the losses between the antenna and rectifier; it does not imply, however, that the entire amount of available power is converted to useful DC power. In fact, RF-to-DC conversion efficiency is the best suitability indicator for the rectifier. where η is the RF-to-DC efficiency, P out and P in are the respective output and input powers of the rectifier, and V out is the output voltage across the load resistance ( R L = 10 K ). The simulated and measured RF-to-DC efficiencies of the rectifier against the operating frequency are portrayed in Fig. 4. Evidently, the peak efficiency depends on the available input power. At 0 dBm, the maximum simulated RF-to-DC efficiencies of 77.02% and 67.04% are found at 915 MHz and 2.45 GHz, respectively. For the same input power (0 dBm), the maximum measured conversion efficiencies of 75.1% and 64% are noticed at 0.915 and 2.45 GHz, respectively. It can also be observed that the peak efficiency drops down with decrease in the input power. At the input power of −10 dBm, the measured maximum efficiencies of 58% and 48.2% are observed at the lower-(915 MHz) and higher-frequency band (2.45 GHz), respectively. Similarly, at the input power of −20 dBm, the measured peak conversion efficiencies of 41.3% and 31.2% are noted at the lower-and higher-frequency bands, respectively. Fig. 5 shows the simulated and measured conversion efficiencies of the rectifier as a function of the input power. In the lower-frequency band (0.915 GHz), the simulated (measured) RF-to-DC conversion efficiency of 79.9% (77.3%) is observed at the input power of 1 dBm. Similarly, for the higher-frequency band (2.45 GHz), it has simulated (measured) RF-to-DC efficiency of 72.8% (66.8%) at 3 dBm. In all frequency bands, efficiencies are greater than 30% at a lower input power of −20 dBm. It is worth mentioning that a single frequency and input power signal is generated at a time and the same procedure is adopted for all other frequencies and input powers. Evidently, the proposed rectifier has the ability to harvest signals with very low amounts of power, thus, making it a suitable choice for power-restricted sources. In fact, all simulations and measurements in Figs. 4 and 5 are carried out at a load impedance of 10 K . It is a fact  www.nature.com/scientificreports/ that RF-to-DC efficiency also depends on the load, thus, a series of simulations are performed at varying load resistance, as shown in Fig. 6. Evidently, the peak efficiency is shifted to lower input powers with increasing the load resistance. Besides, a comparison table (Table 1) is added to clearly identify the advantages of the proposed rectifier compared to other implantable rectifiers. It is worth mentioning that none of the implantable rectifiers in the literature can operate in more than one band. Moreover, the RF-to-DC efficiency of the proposed rectifier is better than the majority of rectifiers.

Dual-band implantable antenna
A meandered-resonator based dual-band implantable antenna is designed and optimized in a full-wave electromagnetic simulator (HFSS). The top, bottom, and side views of the proposed antenna are shown in Fig. 7. It can be seen that the meandered-resonator monopole is located on the top layer of the substrate and that the slottedground plane is located on the bottom layer of the substrate. The width of the meandered line and the gap between them is 0.25 mm. Moreover, a superstrate is added to facilitate loading effects, thus, achieving miniaturization. A high-permittivity material Rogers RO3010 is used as a substrate and superstrate for size reduction. Moreover, open-ended rectangular slots are added in the ground plane to further achieve miniaturization. The proposed antenna is sourced by a 50 coaxial probe. This antenna, along with RF and electronic circuitry, is enclosed in a cylindrical container, as illustrated in Fig. 7b. The cylindrical container is made of alumina material that is 0.2 mm thick. The simulation and optimization of the proposed antenna is performed in the abdomen of the   Design evolution process. The design evolution process of the proposed antenna is shown in Fig. 8, consisting of four steps. Initially, in the first step, a half-wavelength meandered resonator is designed at 2 GHz. It can be observed that the resonance is at 2.2 GHz (Fig. 9). In the second iteration, another stand alone meandered Step 1 Step   Fig. 9. In fact, the open-ended slots at the ground plane induce additional capacitance. Therefore, the resonant frequencies are shifted to the lower-frequency side. Moreover, the addition of capacitive slots on the miniaturization process can be easily understood using slow-wave theory [Equation (5)] 35 . It can be observed that the resonant frequency is mainly dependent on the capacitance and inductance of the antenna. In fact, both capacitance and inductance have inverse relationships with the resonant frequency of the antenna. Thus, increasing the value of capacitance shifts the resonant frequency to the lower-frequency side.
where f r is the resonant frequency of the antenna, v p is the propagation speed, ǫ eff is the permittivity of the substrate, g is the wavelength at f r , L a is the inductance of the antenna, and C a is the capacitance of the antenna.
Simulation and experimental results. The proposed antenna is simulated in the abdomen of the realistic human body, which resonates at 0.915 GHz and 2.45 GHz with a reflection coefficient lower than −20 dB. It can be observed that the antenna covers the desired ISM bands (0.915 GHz and 2.45 GHz) and has a FBW of 14.5% and 8.2% at 0.915 GHz and 2.45 GHz, respectively. To verify the simulated results, the proposed antenna is designed on a Rogers RO3010 substrate. The meandered-resonator lines and slotted ground is designed using chemical etching. Moreover, the cylindrical container is printed using three-dimensional (3D) printing technology. The proposed antenna, RF, and electronic circuitry are placed inside the container and sealed with epoxy, as shown in Fig. 10. Then, the measurements are performed by embedding the capsule device inside a box containing minced pork meat. The reflection coefficient measurements are performed by connecting the antenna with a vector network analyzer (VNA) model E5062A of Agilent Technologies. The measured reflection coefficient is compared with the simulated one in Fig. 11. It can be observed that the measured reflection coefficient has a resonant frequency of 0.916 GHz (lower band) and 2.49 GHz (higher band). Moreover, the measured FBWs are 8.1% and 11.4% at the lower and higher bands, respectively. Figure 12 shows E-plane and H-plane radiation Step 1 Step 2 Step 3 Proposed Figure 9. Reflection coefficients of the dual-band implantable antenna in design iterations.

Wireless power transfer transmitter
A WPT transmitter is a fundamental part for transferring RF power to deeply implanted BIDs. Figure 13a shows the geometry of the proposed log-periodic WPT transmitter. A log-periodic antenna is chosen as a WPT transmitter because of its directional beam and wide bandwidth. The radiating structures of the log-periodic antenna are designed on a 1.28 mm thick Rogers RO3010 substrate. The overall dimensions of the WPT transmitter are 83 × 53 × 1.28 mm 3 . A 50 lumped excitation scheme is used in the simulation. During the simulations, the WPT Transmitter is placed at a distance of 5 mm from the skin of the abdomen, as shown in Fig. 13b. The WPT has wide bandwidth, covering the frequency band from 0.62 GHz to 3 GHz, as shown in Fig. 14. The S 11 measurements of the WPT transmitter are carried out by placing it on side of the minced pork meat and connecting it with the network analyzer. The measured results show a good level of agreement with the simulated ones.

Results and discussion
Initially, the dual-band rectifier, dual-band implantable antenna, and wideband WPT transmitter are individually designed and experimentally validated. In this section, all these parts are integrated to wirelessly power the implant. In fact, the proposed dual-band rectifier is integrated with the dual-band implantable antenna to design the dual-band rectenna. Then, the rectenna and the other electronic and RF components are packed inside a capsule device. Next, the capsule device is implanted in the intestine of a realistic human model, as shown in Fig. 13b. Additionally, the proposed WPT transmitter is placed at a distance of 5 mm from the abdomen of the human model. To determine the PTE of the system, the proposed WPT transmitter is considered as a source and implantable antenna is considered as a receiving unit. The transmission coefficient ( S 21 ) of the proposed WPT system is shown in Fig. 15. The S 21 of the WPT system is −37.9 dB and −23.7 dB at 0.915 GHz and 2.45 GHz, respectively. In fact, the WPT transmitter (outside a human body) operates at different medium than the implantable antenna (inside a human body). Therefore, more power is radiated back from the human body due to the dielectric mismatch. These mismatches can be reduce using high-permittivity matching layers 27 , metasurfaces www.nature.com/scientificreports/ and metamaterials [37][38][39][40] , parasitic patches 25 , and conformal surfaces 41 . In order to minimize such a mismatch, a matching layer of high dielectric constant can be used 27,36 . In this work, a matching layer ( 40 × 40 × 0.1 mm 3 ) of Rogers 3010 ( ǫ r = 10.2) is used. As a result, the S 21 of the WPT system is observed as −20.8 and −18.6 dB at 0.915 and 2.45 GHz, respectively. Consequently, the PTE of the WPT system is improved from 0.07% and 0.43% to 0.83% and 1.3% at 0.915 and 2.45 GHz, respectively. Furthermore, the simulated S 21 results are verified through practical measurements. In measurements, the WPT transmitter is connected to one port of the VNA and the implantable antenna is connected to the second port of the VNA. The practical results agree well with the simulated ones. To verify the wireless powering and RF-to-DC conversion efficiency of the implant, the WPT transmitter is connected to an RF signal generator and output of the rectifier is connected to a digital multi-meter. The capsule implant is placed in the center of a box containing minced pork meat, and the WPT transmitter is placed at a distance of 60 mm from the implant, as shown in Fig. 16. The specific absorption rate (SAR) analysis is performed to ensure patient safety during wireless powering of the capsule implant. The SAR analysis is performed by considering 1-g of tissue and an input power of 1 W (30 dBm), as illustrated in Fig. 17. The capsule device is placed in the intestine of the human body and the WPT transmitter is placed at a distance of 5 mm from the abdomen. At an input power of 1 W, the peak SAR values of 1.22 and 0.90 W/Kg are observed at 0.915 and 2.45 GHz, respectively. The SAR values in both bands are lower than the standard provided by the Institute of Electrical and Electronics Engineers (IEEE). The standard limit for 1-g tissue is 1.6 W/Kg for an input power of 1 W; thus, the proposed WPT system is suitable for the wireless powering of the biomedical implants using 0.915 and 2.45 GHz frequency bands. Table 2 shows a comprehensive comparison of the proposed WPT system with state-of-the-art WPT systems. The unique advantage of the proposed WPT system is that it can efficiently rectify two RF signals, while all other WPT systems can only harvest one RF signal. Other advantages of the proposed system include higher PTE in www.nature.com/scientificreports/ both bands, the compact dimensions of the implantable antenna, and the implantable antenna's higher gains. Moreover, the proposed WPT transmitter has a wideband, which can be used as a transmitter for any system operating between 0.62 and 3 GHz.

Conclusion
In this paper, a dual-band implantable rectenna has been simulated and practically validated for recharging and operating medical implants at 0.   www.nature.com/scientificreports/ GHz, respectively. Similarly, a dual-band rectifier is simulated and practically demonstrated at 0.915 GHz and 2.45 GHz. The dual-band implantable antenna is integrated with the dual-band rectifier to form a dual-band rectenna. The rectenna, electronic and RF components are packed inside a capsule device and implanted in the intestine of the realistic human model. At an input power of 0 dBm, the rectifier has RF-to-DC conversion efficiency of 77.02% and 67.04% at 0.915 GHz and 2.45 GHz, respectively. A wideband WPT transmitter is also designed, which operates between 0.62 and 3 GHz, to provide an RF signal to the rectenna. After each component of the WPT system is individually simulated and practically validated, finally, as a proof, the WPT transmitter is sourced with an RF signal generator and the rectenna is placed inside minced pork meat and the received voltage is observed on the DMM. Using a matching layer, the PTE of the WPT system is improved from 0.07% and 0.43% to 0.83% and 1.3% at 0.915 GHz and 2.45 GHz, respectively. Furthermore, it has a SAR at both frequency bands, where the peak SAR is 1.22 W/Kg at 0.915 GHz and 0.90 W/Kg at 2.45 GHz. The proposed implantable rectenna shows good performance in terms of its dual-band operation, human safety analysis, high RF-to-DC conversion efficiency, and compact dimensions. Moreover, this is the first implantable rectenna that performs dual-band RF signal rectification.